VANCOUVER, Washington (CNN) -- Scientists are closely watching Mount St. Helens after a small eruption spewed smoke and ash thousands of feet Friday before another earthquake rattled the volcano.

A series of small earthquakes was detected in the week before the Friday afternoon's eruption. This seismic activity decreased shortly after the noon (3 p.m. ET) eruption, but picked up again within hours.

Peter Frenzen, a scientist with the Mount St. Helens National Volcanic Monument, said a 2.0 magnitude earthquake was detected.

One scientist described the eruption as a "hiccup."

The volcano spewed a harmless plume of steam and ash into the air Friday, the biggest eruption by the volcano in 18 years.

A small explosion was detected on the south side of the volcano's lava dome, where cracks had been detected in a glacier, said John Major of the U.S. Geological Survey.

The mountain then vented a combination of steam and ash for 24 minutes, sending a pale gray column nearly 10,000 feet into the blue Washington sky.

"There is no indication that magma has reached the surface," Major said.

Molten rock is called magma before reaching the surface where it then becomes lava.

Scientists said the presence of magma could indicate the potential for a more serious eruption.

Geologist Tom Pierson said the event "was really a hiccup. [Eruptions] could be a little bigger once real magma is involved."

A visible plume -- which was mostly steam but contained some ash -- moved southwest about six miles, Major said.

The Portland, Oregon, metropolitan area lies about 50 miles southwest of Mount St. Helens.

The water flow out of the crater appears to have increased since the eruption, though no potentially destructive mud flows were reported, he said.

Scientists saw this coming

Scientists had been predicting just such a minor eruption after swarms of small earthquakes were detected and the mountain's volcanic dome shifted three inches since Monday.

"This is exactly the kind of event that we've sort of been talking about and anticipating over the past several days. This is a fairly small eruption," Major said.

In anticipation of an eruption, the mountain was closed to hikers, and the media and sightseers gathered at a visitors center five miles away.

Major said none of the scientists working on the volcano at the time of the eruption were injured.

Friday's eruption was a mere sideshow in comparison to the cataclysmic eruption May 18, 1980, which blew off more than 1,000 feet from the top of the mountain.

That eruption killed 57 people, left deep piles of ash hundreds of miles away and caused $3 billion in damage.

After that disaster, small eruptions continued at Mount St. Helens until 1986, when the volcano finally went quiet. Major said Friday's eruption was comparable to the minor eruptions seen during that period.

The lava dome was built up inside the crater left by the 1980 eruption by the smaller eruptions that followed it.

Flights delayed

The Federal Aviation Administration temporarily diverted an unknown number of commercial aircraft after the eruption, primarily affecting air traffic in the Portland area, spokesman Allen Kenitzer said.